1. Field of the Invention
This invention relates to the field of testing semiconductor devices for defects, and more specifically, to the evaluation of methods for detecting defects in a semiconductor device using IDDQ testing in the presence of high background leakage current.
2. Background Information
In the manufacture of semiconductor devices it is important to have methods for testing a device and determining if it is a good device or if it contains defects before selling such a product to a customer. If the results of the test are within a manufacturer's tolerance levels then the device is presumably a good or non-defective device and may be sold to a customer. If the results are not the same and are not within the manufacturer's tolerance levels then the device is a defective device and cannot be sold to a customer.
One particular method for testing a device for defects is called IDDQ Testing. In a CMOS device when the clock is stopped, the device is said to be in a quiescent state, thus the current in the device is called drain to drain quiescent current (IDDQ). IDDQ derives from quiescent IDD which is the current drawn by the Vdd power supply. The Vdd supply is typically held at a voltage above ground and fixed within narrow bounds. The other supply is typically called Vss and is taken to be ground (i.e. arbitrarily assigned a value of zero electrical potential. In IDDQ testing, a device is tested by measuring the current while the device is in the quiescent state. Since defects often result in significant leakage currents, measuring the quiescent current allows defects such as open and short circuits to be detected. If the IDDQ is above a preset threshold, then the device is termed "defective" and is not sold to the customer.
Prior methods for IDDQ testing require a low background leakage current while in the quiescent state. In such prior methods, the background leakage current is typically required to be less than a few hundred microamps (.mu.A). As is illustrated in FIG. 1, a statistical sample of devices under test (DUTs) is taken in order to determine a median background leakage current 110. Once a median background leakage current is established then an IDDQ pass/fail limit 120 is set. The pass/fail limit must be set such that it is greater than the median background leakage current but such that it is less than the average current caused by a device defect. Typically in prior methods the pass/fail limit is set at a current much higher than the median, usually from three (3) to six (6) standard deviations greater than the median background leakage current. For example, the pass/fail limit may be in the range of approximately 500 microamps (.mu.A) to 1.5 milliamps (mA). Any device exhibiting an IDDQ current greater than the pass/fail limit is assumed to be a defective device 130 and is not sold to a customer.
One of the problems with prior art methods for IDDQ testing is that they can only detect defects where the defect causes an IDDQ current larger than the background leakage current. As device dimensions of semiconductor devices become smaller and more dense, the background leakage current increases in relation to the defect currents which must be resolved.
This increase in background leakage current can be traced to several factors. The most prominent of these are the increase in the number of devices on a single substrate and an increase in the subthreshold leakage across a given device as the length of its polysilicon gate decreases. The latter, in turn, can be divided into several components, each of which contributes a share to the total current. For example in submicron CMOS devices, background leakage currents in the range of approximately several tens of milliamps (mA) are likely due to the short channel lengths.
As is illustrated in FIG. 2, a wider distribution in background leakage current makes resolution of the same level of defect current problematic. Since the background leakage is higher, the pass/fail limit 220 must be increased proportionately to avoid the improper rejection of functional devices. Meanwhile, the standard deviation of the background leakage 230 has increased sufficiently to make resolution of smaller defect currents impossible.
Another problem with modes of IDDQ testing is that there is no real way to determine how accurate a method may be at detecting defects. There is also no way to compare one method for IDDQ testing to another since in many of the testing techniques the derived measures which serve as the pass/fail criteria are substantially different from each other. For example, one derived measure may be a unitless ratio of currents, whereas another may be a current itself measured in milliamps. Thus, it is difficult to compare the results of such different techniques without a common standard.
Thus, what is needed is a method for detecting defects in a semiconductor device in the presence of a high background leakage current where the accuracy of the method may be determined and the pass/fail limit optimized. This can save cost and increase quality by increasing the number of defective devices detected and decrease the number of good devices thrown away.